1. Field of the Invention
The present invention is related to a materials system wherein a bimetallic product is produced by diffusion bonding a low alloy steel with austenitic stainless steel. More particularly, the invention relates to such a materials system produced by diffusion bonding at an elevated temperature, followed by austenitizing, cooling, and aging of the resulting combination to produce a composite material with desirable metallurgical and mechanical properties, and the virtual elimination of carbon transport across the diffusion bond.
2. Description of the Prior Art
Bimetallic products, especially bimetallic tubing, have been used to take advantage of the specific properties of more than one material in a single product. In such products, a material with one desirable property is bonded to another material with a different property. U.S. Pat. No. 3,566,741 (Sliny) shows a tubular, seamless, dual hardness armor plate utilizing a metallurgical bond between a ductile inner shell and an impact resistant outer shell. The resulting advantage is superior impact resistance to projectiles, drilling tools, etc. U.S. Pat. No. 3,693,242 (Chivinsky) shows the use of carbon steel and stainless steel as components of a composite material in which heat conducting qualities of the carbon steel are combined with a stain resistant quality of the stainless steel. U.S. Pat. No. 3,696,499 (Dromsky) discloses an arrangement in which a layer of stainless steel is sandwiched between and metallurgically bonded to two layers of low carbon steel. The resulting composite tube is suitable for use. e.g. in brake tubing. Inclusion of a stainless steel layer provides improved corrosion resistance.
One combination of materials involves the use of austenitic stainless steel with a low alloy steel to produce a bimetallic tube. Typically, a thin layer of stainless steel provides the corrosion resistance; the low alloy steel provides high strength at low cost. Unfortunately, metallurgical problems arise in the production of such a product.
First, because of the large difference in carbon activities between the two materials, during all hot working and heat treating operations carbon will tend to diffuse from the low alloy steel to the austenitic stainless steel. This results in a reduction in the desired properties of both materials. Because carbon is usually a very important factor in determining the hardenability of the low alloy steel, the loss of carbon by this phenomenon can lead to reduced strength in this material. The corresponding increase in carbon content in the stainless steel results in chromium carbide formation at grain boundaries during cooling with attendant reduction of chromium at those boundaries and thus reduced corrosion resistance. This problem becomes especially severe in the case of an optimally designed bimetallic product where only a very thin layer of stainless steel is used; here the entire layer can become carburized, rendering the product unfit for its intended service.
One solution is the use of a properly selected interface material, for example nickel which when positioned between the low alloy steel and the stainless steel will limit the problem by presenting a barrier to carbon diffusion. However, this approach significantly increases the cost and complexity of the bimetallic product and makes other fabrication operations with the bimetallic product such as welding, more difficult.
A second problem arises from the fact that the austenitic stainless steel/low alloy steel combination is generally not compatible in heat treatment. For optimum corrosion resistance the austenitic stainless steel requires a solution anneal in which it is heated to at least 1850.degree. F. and water quenched. After such a treatment it is very important that this material not be heated for extended periods in the temperature regime of 950.degree. F. to 1500.degree. F. in order to prevent "sensitization", i.e., chromium carbide formation at grain boundaries with attendant loss of corrosion resistance. The low alloy steel requires a quench and tempering treatment in which the material is heated to about 1650.degree. F., water quenched, and tempered at between 1100.degree. and 1200.degree. F. for one or more hours in order to provide optimum mechanical properties. The tempering portion of the cycle imparts good fracture toughness to the low alloy steel; however this same treatment tends to cause "sensitization" of the austenitic stainless steel and loss of corrosion resistance. Since the sensitization phenomenon increases in severity with increasing time and/or temperature, the choice of a low temperature and/or a very short tempering time should offer some relief from this problem. However, short times and low temperatures in general result in poor fracture toughness for the low alloy steel. Since increasing carbon content tends to accelerate the onset and the severity of the sensitization phenomenon in the austenitic stainless steel, use of low carbon, "L grade", stainless steel (e.g. 304L with a maximum of 0.035% carbon) appears to afford some relief. However, the lower carbon content of the austenitic stainless steel will only accentuate the carbon activity gradient relative to the low alloy steel (with a typical carbon content of about 0.40%) and worsen the carbon diffusion problem cited above. In the past, an appropriate heat treatment was selected to provide optimum properties for one material, while tolerating minimal but not totally degraded properties in the companion material. Alternatively, the combination was treated such that only moderate properties would be achieved in both materials.